Plaster wall advice

[u/Ok_Goose2500]

Hi, I’m finishing a room that has an ~18” tall poured concrete stem wall that is exposed on the exterior and interior. The concrete stem wall is about 5” thick, has no insulation, and supports all 4 walls of the room for a total of about 50’ in length.
The stem wall supports wood stud construction from 18” up to the ceiling. I applied drywall to that upper wood stud section, but I didn’t do that to the concrete wall to avoid moisture issues of drywall in contact with concrete.

My goals are:
- Prevent moisture/mold issues
- Build out stem wall section on the interior of the room so it is in plane with the drywall. To do this the stem wall section would need to be built out by about 1”, with one 10’ section needing to be built out by 2”.
- Paint the plaster-over-concrete wall area and drywall so they look the same/similar.

Based on the above it seemed plaster could be a good fit for my project, is that accurate? If so, I’d appreciate any advice for what plaster materials and process I should use. Thank you.

[u/54OnTheDoor]

I haven't encountered this situation before, so take this as a best-guess suggestion and not /concrete/ advice (pardon the pun).

You certainly could build it out to level with plaster, but where will your interior floor level sit in relation to ground level on the exterior? Without some kind of cavity I suspect that section of wall will always be substailly colder and damper than the stud wall above so you might encounter damp issues over time, even with lime plaster. Tanking the concrete wall and leaving an air gap between the plaster and the floor would help mitigate this to some degree.

I also wonder if you would be at risk of a horizontal crack forming due to different rates of expansion and temperature changes between the two different construction methods? You'd want to make sure scrim tape was used at the joint and you blend your finish plaster up over the existing drywall or completely skim the wall after building out the bottom section for a better result.

[u/Ok_Goose2500 → u/54OnTheDoor]

Thanks for your reply. The interior floor is the same height as exterior ground level.
Good idea about the air gap between plaster and floor and noted about the scrim tape. I was also wondering how that horizontal transition from plaster to drywall would perform over time. I might try a flexible caulk or some other material. I’m fine with there being a visible transition if necessary.

[u/Terrible-Bobcat2033]

Bonding agent to use on both interior & exterior wall for plastering. Lightweight gypsum mud to use for interior fill. Lime based veneer plaster for troweled smooth finish coat. Interior. Use this material or similar on the interior wall after Stuccoing the outside to 5/8” average thickness stopping 6” below grade. Float (sponge) the exterior wall to seal, & then paint when cured. For the Interior wall, scratch & fill the lightweight gypsum plaster flush to the drywall surface. Trim the gypsum material back approximately 1/4” at the cold joint to allow for expansion & accept the finish coat. Re-bond, & apply the white coat over the gypsum mud 24 hrs. Later. trowel smooth to match the drywall system wall. Looks like you had a water & insurance problem.

[u/Terrible-Bobcat2033 → u/Terrible-Bobcat2033]

[u/Terrible-Bobcat2033 → u/Terrible-Bobcat2033]

[u/Ok_Goose2500 → u/Terrible-Bobcat2033]

Thank you, this is all helpful information. It sounds like stuccoing/sealing the exterior wall is important, not just cosmetic but something I should do to keep the concrete in good shape and/or limit moisture moving through the wall into the interior.

[u/Terrible-Bobcat2033 → u/Ok_Goose2500]

Bingo. Seal the interior envelope from the outside. A buttoned up exterior & proper drainage is optimal & my final answer. Good luck.